1. Field of the Invention
The present invention generally relates to the art of consumable electrode arc welding and, more particularly, to a method of controlling the apparent length of the welding arc between the workpiece and a consumable electrode to a constant value during the practice of the arc welding process.
2. Description of the Prior Art
To secure the stabilized high-quality weld during the practice of the consumable electrode arc welding, it has long been well known that the minimum mean distance La (which is hereinafter referred to as "apparent arc length") between the tip of a consumable welding wire and the workpiece has to be maintained at a constant value. It is also well known that in welding the workpiece made of aluminum or steel, weld defects such as undercuts and/or blowholes tend to occur if spray transfer of molten metal droplets is carried out with the welding arc of such a small length that no short-circuiting take place.
Accordingly, to avoid the weld defects such as discussed above, the welding method has hitherto been employed in which the welding is carried out by generating short-circuiting which is synchronized with the pulsing current at a proper arc voltage, but which is too minute to initiate metal transfer of molten metal droplets towards the workpiece. FIG. 1 illustrates diagrammatically welding situations in which the minute short-circuiting occur when the apparent arc length is controlled to a proper value. In the practice of the standard pulsed arc welding, the minute short-circuiting tends to occur in synchronism with metal transfer at a frequency matching with the pulse frequency f and, therefore, the number of short-circuits occurred (which number is hereinafter referred to as "short-circuiting frequency") is high at a large current region in which the pulse frequency f is high as shown in FIG. 1A, but is low at a small current region in which the pulse frequency f is low as shown in FIG. 1B.
As discussed above, change of the average welding current Ia tends to result in change of the pulse frequency and, hence the cycle of minute short-circuiting changes with change of the pulse frequency.
In view of the foregoing, the arc length controlling method has been employed in which in order to maintain the constant apparent arc length La, the output voltage Vt is controlled so that the short-circuiting frequency counted for a given length of time can attain a predetermined target value.
However, the prior art arc length controlling method has the following problems.
Referring to FIG. 2, a comparison chart is given to illustrate how the arc length La changes in each of small, medium and large current regions with change in arc voltage and welding current when the target or desired short-circuiting frequency Nr and the length of time (or, counting time) Tr during which the short-circuiting frequency is counted are fixed to a predetermined value. As shown therein, the counting time Tr and the target short-circuiting frequency Nr in each of the small, medium and large current regions are fixed to 1 second and 5 times, respectively.
The comparison chart of FIG. 2 will be described with particular reference to FIG. 6 which illustrates the relationship between the average welding current Ia and the arc length La which are exhibited when the welding is effected with the use of the prior art method and the arc length controlling method of the present invention, respectively.
When the welding current I is changed from 50A to 300A while the target short-circuiting frequency Nr and the counting time Tr are fixed at a predetermined value, the prior art method has resulted in that the apparent arc length La has considerably changed from 1.5 mm to 5.5 mm with change in welding current.
As shown in FIG. 2, the frequency of occurrence of the peak currents Ip during the counting time Tr and the frequency of occurrence of the minute short-circuiting Psh have the following relationship.
(1) In the small current region, the frequency of occurrence of the minute short-circuits is 5 while the frequency of occurrence of the peak currents is 3. Thus, in the small current region, the frequency of occurrence of the minute short-circuits is greater than the frequency of occurrence of the peak currents. This means that the apparent arc length La is small as shown by Ls.
(2) In the medium current region, the frequency of occurrence of the minute short-circuits is 5 while the frequency of occurrence of the peak currents is 5. Thus, in the small current region, the minute short-circuiting Psh occurs in synchronism with the peak current. This means that the apparent arc length La is of a proper value as shown by Lr.
(3) In the small current region, the frequency of occurrence of the minute short-circuits is 5 while the frequency of occurrence of the peak currents is 8. Thus, in the small current region, the frequency of occurrence of the minute short-circuits is smaller than the frequency of occurrence of the peak currents. This means that the apparent arc length La is long as shown by Lt.
For the reason discussed above, as shown in FIG. 6 in connection with the prior art, and in the small current region ranging from 50A to 100A, La&lt;Lr; and in the large current region ranging from 250A to 300A, Lr&lt;Lt. Thus, depending on the small to large current regions, the apparent arc length changes in the order of Ls&lt;Lr&lt;Lt.
In the practice of the prior art arc length controlling method, the output voltage Vt is so controlled that the short-circuiting frequency Nk counted during the fixed counting time Tr can attains a predetermined target value Nr. However, as discussed above, increase or decrease of the average welding current Ia with increase or decrease of the pulse frequency f tends to result in change of the apparent arc length and, therefore, in all of the small to large current regions, the apparent arc length La cannot be controlled to a proper value.
As shown in FIG. 7B, the prior art controlling method has resulted in insufficient bead penetration and weld defects such as undercuts and/or blowholes, which have occurred at the large current region.